A large number of catalyst materials exhibit acidic sites on the surface and within the pores of the catalyst. Specific catalyst support materials displaying this property include synthetic aluminosilicates, natural aluminosilicates, silica gels, and alumina. Preferred catalysts useful in the invented process are crystalline aluminosilicate or borosilicate zeolites termed molecular sieves.
Molecular sieves are known to have uniformly structured pores in which ion exchange sites exist. These sites are readily made into the acid form by known techniques and thus provide a highly active acidic site within the geometric constraints of the sieve pore. This combination of acidity and geometrical constraint yields a catalytic site which tends to be very active for certain acid catalyzed reactions but inactive for other reactions which are more sterically demanding.
Since the molecular sieve is originally made in the form of a fine powder, some method of supporting it is required as larger particles must be utilized to allow its use in commercial reactors. However, many supports, while binding the sieve into usefully sized particles, also contain intrinsic acid sites of their own. These support acid sites can give rise to undesirable acid-catalyzed reactions which would not be catalyzed by the sieve acid sites. A common method of neutralizing acidic sites on a catalyst is by exchange with an alkali metal such as sodium or potassium. However, any aqueous alkali metal solution would exchange both the support and the sieve sites rendering the catalyst inert for acid catalyzed reactions.
A process has been found for selective exchange of undesirable acidic sites on the surface of a catalyst containing both a support and a molecular sieve. This process is selective in that while exchanging the support acid sites causing cracking activity, it does not extensively neutralize the acidic sites which catalyze dehydrogenation and isomerization reactions. The selective exchange of undesirable acidic sites apparently can be used to neutralize these acidic sites so that the resultant catalyst will contain only desirable active sites. Selective exchange of undesirable acidic sites can also be used to place an active component only on the catalyst support to yield a bifunctional catalyst.
Modification of the activity of catalysts employing certain crystalline aluminosilicate zeolite compounds to improve selectivity of the resulting catalytic reaction is known in the art. However, these modifications have been directed to modification of the crystalline aluminosilicate and borosilicate compounds and not to support material, if such is part of the catalyst composition. For example, U.S. Pat. No. 3,251,897 teaches use of an aluminosilicate catalyst that has been metal base exchanged or hydrogen acid exchanged, or both, so as to have a minimum level of catalytic activity, thus producing a high level of alkylation products particularly at low temperatures both in liquid and mixed phases. This minimum level of activity is taught as depending upon the degree of exchange of the metal from the sites within the aluminosilicate catalyst either with the base exchanged metal, or in the case of acid exchanged, with hydrogen or both. U.S. Pat. No. 2,904,607 teaches the reaction of alkyl aromatic compounds by reacting aromatic and olefinic hydrocarbons using a crystalline non-acidic aluminosilicate catalyst having pore openings adequate to admit freely the individual aromatic and olefinic molecule. The pore openings are from 6 to 15 .ANG., i.e., too large an opening not permitting high activity because of concomitant decrease in surface area. Pore size openings can be modified by base-exchanging the prepared sodium zeolite catalyst with another ion, such as calcium, to form a calcium sodium aluminosilicate. U.S. Pat. No. 3,682,996 describes crystalline aluminosilicate zeolites modified by reaction with an organic substituted silane. The preferred zeolite species has a pore size greater than 7 .ANG. in diameter, since such pore size will readily accommodate the organosilane into the porous structure of the aluminosilicate. The modified catalytically active zeolite catalyst can be employed in certain shape selective catalyzed reactions since, due to the substantial decrease in sorption properties, only molecules of special size will pass through the zeolite and undergo catalytic change. U.S. Pat. No. 3,698,157 describes an improved method for separation and isolation of individual components in a C.sub.8 aromatic mixture by contacting the mixture with an aluminosilicate zeolite which has been modified by contact with an organic-radical substituted silane. U.S. Pat. No. 4,100,215 describes preparation of a zeolite catalyst which has been contacted with a surface modifying agent capable of deactivating catalytic sites located on the external surface of the zeolite. Treatment involves contact with a suitable compound of silicon or nitrogen of a size sufficiently large to be unable to penetrate the zeolite pore structure. Representative of these compounds are phenyl carbazole, dimethyl dichloro silane, trimethyl chlorosilane, substituted phenyl acidines, and organic substituted silanes. The resulting catalyst is useful in a process for the selective production of paraxylene by methylation of toluene.
While the above-mentioned prior art is considered of interest in connection with the subject matter of the present invention, the selective process described herein for selective exchange of acidic sites on the surface of the support of the zeolite sieve component has not, insofar as is known, been heretofore disclosed. The resultant catalyst selectively isomerizes hydrocarbons with reduced by-product formation. Selective exchange of support acidic sites can be used to place an active component primarily on the catalyst support to yield a bifunctional catalyst.
It is an object of the present invention to provide an improved catalyst composition comprising a crystalline aluminosilicate or borosilicate molecular sieve and a support material wherein acid sites on the surface of the support are exchanged with an organo metal compound while the acid sites of the zeolite are not completely exchanged.
It is an object of this invention to provide an improved catalyst composition comprising a crystalline aluminosilicate or borosilicate molecular sieve and a support material wherein the acid sites on the surface of the support material are exchanged with an active component while the acid sites of the zeolite are not completely exchanged, thus providing a bifunctional catalyst.
It is an object of this invention to provide a process for selective exchange of acidic sites on the surface of the support material in a catalyst containing both support material and a crystalline aluminosilicate or borosilicate molecular sieve while the acid sites of the zeolite are not completely exchanged.
Other objects will appear hereinafter.